1. Field of the Invention
The invention relates to a latch system, and more particularly, to a latch system comprising an action module for increasing the negative resistance of the latch system.
2. Description of the Prior Art
Among electronic circuit design for communications and other varieties of applications, voltage-controlled oscillators (VCO) have always been one of the most important circuit building blocks. Usually, a phase lock loop (PLL) is used in a VCO for generating a clock signal with fixed phase, and the clock signal could then be used for transforming high frequency signals into low frequency signals in communications applications. In other words, the PLL is used as a local oscillator (LO). In general, there are two mainstream design methodologies for VCOs in conventional complementary metal-oxide semiconductor (CMOS) fabrication technologies: one is to use only standard CMOS fabrication technologies, for example relaxation oscillators and ring oscillators; the other one is to use LC tank structure in an oscillator circuit. The later one tends to increase the degree of complexity in a manufacturing process for its utilization of integrated inductors in CMOS fabrication technologies, but since it has a relatively much simpler circuit structure at the same time it will generate clock signals with much higher quality (for example more independence from variations in supply voltages and temperature, as well as higher immunity against noise). Therefore, VCOs with LC tank structure are still favorable for most of the circuit designers.
Please refer to FIG. 1 that shows a block diagram of a prior art LC tank VCO 10. The VCO 10 comprises a current source module 24, which is used for providing a DC current; a cross-coupled pair 12, which is usually formed by two exactly identical MOSFETs 14 and 16. A drain of the MOSFET 14 is electrically connected to a gate of the MOSFET 16, while a drain of the MOSFET 16 is electrically connected to a gate of the MOSFET 14, and signals are taken out from the two gates (which are also the drains in this example) as two output nodes of the VCO 10. A resistance RN shown in FIG. 1 represents the equivalent resistance of the cross-coupled pair 12. The VCO 10 also comprises a LC tank 18, which includes a capacitance 20 and an inductance 22, and is electrically connected between the two output nodes of the VCO 10. A resistance RP represents the equivalent resistance of all the elements in the LC tank 18. From the circuit structure mentioned above, the VCO 10 will be able to start with a tiny noise signal existing in it, without any help from external initial signals, through the oscillation of the cross-coupled pair 12 accompanied with the capacitance 20 and the inductance 22 with respect to the tiny noise signal, to generate two oscillating signals with 180° phase difference at the two output nodes, while the oscillating frequency of the signals is related to the natural frequency of the LC tank 18. Please note, in FIG. 1 the capacitance 20 of the LC tank 18 represents the equivalent capacitance of the LC tank 18, and the inductance 22 represents the equivalent inductance of the LC tank 18. Usually the LC tank 18 of the VCO 10 will have a voltage-controlled mechanism in order to control the oscillating frequency of the VCO 10, but for simplification, the voltage-controlled mechanism is not shown in FIG. 1.
For the VCO 10 to enter oscillation successfully, it must meet ascertain criterion, which is that the sum-up equivalent resistance has to be a negative value, because only a circuit structure with a negative equivalent resistance will have the ability to enlarge a tiny signal. For the VCO 10 in FIG. 1, its sum-up equivalent resistance could be seen as the add-up of the equivalent resistance of the LC tank 18, RP, and the equivalent resistance of the cross-coupled pair 12, RN. Usually, the equivalent resistance of the LC tank 18 RP is a positive value. This means, if the VCO 10 would like to enter oscillation successfully, the absolute value of the equivalent resistance of the cross-coupled pair 12 RN must be larger than the resistance RP. Assume that the transconductances of the MOSFET 14 and 16 are both gm, the equivalent resistance of the cross-coupled pair 12 RN will be a negative value (−2/gm). This is exactly the reason why the VCO 10 could enter oscillation.
However, because of the applications such as a wireless local area network (WLAN), the frequencies of oscillating signals in present day circuit designs tend to have very high values (for example 5.2 GHz or above). Also, in order to control the cost of manufacturing, the quality factor Q of the inductance 22 is hard to improve. These factors cause the absolute value of the equivalent resistance RN to be larger than the value of the equivalent resistance RP an extremely difficult goal to achieve, and as a result, the VCO 10 is very hard to enter self-resonance. The VCO technologies according to prior art usually will obtain a larger absolute value of the equivalent resistance by raising the DC current of the current source module 24 used as the bias current of the VCO 10 in order to lower the transconductances of the MOSFETs 14, 16. However, if this is the case, the power consumption of the VCO 10 will not be able to be constrained, and thus it is not suitable for the low power dissipation requirement of the present day communications circuit design.